Quantum communications established between ground and space

Using a specialised satellite launched year, Chinese scientists have achieved communication over a record-breaking distance using entangled photons in a major step towards hack-proof communication

When pairs of particles are created at the same instant, these twin particles become ‘entangled’. Even if the particles are at opposite ends of the universe, changing the state of one will cause the state of the other to change instantaneously.

Albert Einstein was reluctant to believe the counterintuitive phenomenon could be true, and referred to it as “spooky action at a distance”.

A range of fantastical possibilities could become reality if quantum entanglement could be harnessed by scientists in practice. In theory, this could include absolutely secure communications. A pair of people wanting to communicate with no risk of interception would carry out measurements on pairs of entangled photons to create a random ‘key’, used to decrypt each other’s messages. If a third person attempts to intercept the communications, their eavesdropping will disrupt the photons, and the sender and receiver will know their messages are being intercepted.

This technology is “the only way to establish secure keys between two distant locations on earth without relying on trustful relay”, said Professor Jian-Wei Pan, a professor at the University of Science and Technology of China, and a leading expert in the field of quantum entanglement, who led the project.

Such a secure and effective quantum communication network relies, however, on the ability to distribute entangled photons between receiver stations separated by large distances. Previously, scientists have only managed to communicate over line-of-sight links across cities or between mountaintops, due to scattering and coherence decay causing channel loss.

The Chinese team, Quantum Experiments at Space Scale, have had their sights set on increasing this distance for more than a decade, and in August 2016 the Micius satellite was launched into orbit. This satellite is loaded with quantum optical equipment, specifically designed to establish secure quantum communications between ground and space.

The Pentagon described the launch of the experimental satellite as a “notable advance in cryptography research”.

The team distributed entangled pairs of photons over the enormous distance, improving on the previous record of 100km by more than an order of magnitude.

This required the scientists first to carry out extensive ground-based tests to ensure that the stream of photons would be able to survive the journey through the turbulent lower atmosphere. Next, they tracked the satellite as it moved over China in the same passage each night for just 275 seconds, and locked onto it using three optical telescopes.

By transmitting photons through space rather than through optical fibres, they were able to transmit many pairs of photons every second. According to the researchers, this allowed them to increase the distribution efficient by 12 orders of magnitude.

Their successful experiment opens up “bright prospects” for hack-proof communications, Professor Pan commented. In their Science report, the team conclude that this “[illustrates] the possibility of a future global quantum communication network.”

China’s national space programme has been accelerated under President Xi Jinping, who has argued that investment in space research is important for national security and defence purposes.